The invention is an improvement upon the hydrodynamic torque-transfer unit disclosed in U.S. application Ser. No. 208,787, filed Nov. 20, 1980, now U.S. Pat. No. 4,343,383 incorporated herein, and the corresponding German applications on which that U.S. application is based. The previous disclosure is of a hydrodynamic torque-transfer unit, especially a hydrodynamic brake, that seeks to obtain the highest possible ratio between the largest and smallest transferable torque and that will consequently react very rapidly to a command to change from idle to low torque. The unit of the previous disclosure attains this object by conveying a short, temporary filling stroke to the working fluid chamber of the hydrodynamic unit when such an activating command is received. This filling stroke is automatically started by a control device that activates the intake valve to the working fluid chamber of the unit, with the duration of the stroke being precisely varied in accordance with momentary torque requirements. It is thus possible to very rapidly fill the working chamber with working fluid to only a very slight extent when necessary. This results both in high responsiveness and in very low minimum torque, i.e. minimum braking moment, in the case of a brake.
The above application discloses various embodiments of the hydrodynamic torque-transfer unit described above. The following characteristics are common to all of thess embodiments. Each has an activating control, a pedal, for example, which provides a nominal value to a working chamber outlet-control that establishes the necessary torque. The activating control is preferably designed in accordance with DE-OS 28 55 654, corresponding to U.S. Pat. No. 4,276,970. The activating control is also coupled to a two-stage pilot device that opens the intake valve. This system is designed so that when the pedal is slightly depressed, pressure is applied to a pressure-medium line ("Stage I" of the activating command) and, as the pedal is further depressed, pressure is applied to a second pressure-medium line ("Stage II" of the activating command).
The unit of the previous disclosure has been tested with positive results. The system having the pilot device is, however, comparatively expensive and is subject to failure under certain conditions.